1. Field of the Invention
The present invention relates to a measuring cell. More specifically, the present invention relates to a measuring cell with a base body, a measurement membrane arranged on the base body, and a measurement device, with a clearance between the measurement membrane and the base body filled with a fluid which presents an increased heat capacitance (κ) compared to air.
2. Description of the Related Art
The related art involves pressure measuring cells. Such measuring cells are known in the state of the art, for example, as pressure measuring cells for the capacitive detection of a pressure applied externally to the measuring cell. Such a capacitive pressure measuring cell presents a base body, and a measurement membrane arranged on the base body, where, on facing surfaces of the measurement membrane and of the base body, laminar electrodes are arranged to form a capacitance. When pressure is applied to the measurement membrane, the separation between the measurement membrane and the base body changes. Thus, the capacitance of the capacitor formed by the measuring electrodes changes, so that it becomes possible to detect an applied pressure.
In measuring cells constructed according to this principle, it is problematic that, due to rapid temperature changes, so-called thermal shocks, for example, a pressure jump from 20° C. to 80° C., and because of a relatively slow temperature equalization within the measuring cell, structure-mechanical deformations of the measuring cell occur, which, due to the resulting bending of the measurement membrane, produce a change in the measured value, although no pressure change has occurred. If the temperature changes are slow, the temperatures equalize due to heat conduction via a glass solder connection between the measurement membrane and the base body, so that no deformations of the measuring cell and particularly of the measurement membrane are caused. However, in case of rapid temperature changes, such as those that occur, for example, during thermal shock, the temperature equalization between the measurement membrane and the base body occurs only after the longer time, so that, due to the temperature gradient, the membrane undergoes a deformation with respect to the base body, the capacitance conditions inside the cell change, and a pressure change is simulated.
By compensating for such erroneous measurements, it is known, in the state of the art, (as is taught, for example in EP 1 186 875 B1), to arrange a temperature sensor for the acquisition of temperature changes in a glass solder connection, by means of which the measurement membrane is arranged on the base body. This temperature allows distinguishing between temperature changes with a steep temperature gradient and actual pressure changes, and compensating the issued values with the help of electronic processing.
Additionally, attempts have been made to use a bending line of the measurement membrane, and the resulting changed capacitance value, to detect the occurrence of a thermal shock and to correct the incorrect measured value.
What is not appreciated by the prior art is that the presence of a thermal shock can only be detected from incorrect measured values, and consequently a very rapid signal processing of the measured values is needed. Another problem is that it is not possible to reliably detect temporally overlapping events, such as, for example, the overlap between a pressure pulse and a thermal shock, and process it.
Accordingly, there is a need for an improved measuring cell that operates in such a way that the effects of thermal shocks and the resulting measurement errors are reduced.